Phrenology is a theory, popularized in the early 19th century, that links the shape and measurement of the skull to personality traits. Phrenology was met with skepticism even in its early days and is now considered a pseudoscience, developed through improper application of the scientific method. However, modern day scientists recognize phrenology as a precursor to more widely accepted theories about the brain.
Phrenology was the first major theory to recognize the importance of the brain and to postulate that different areas of the brain regulate different functions. Similar ideas were discussed as early as the classical era, for example in the writings of Aristotle, but Franz Joseph Gall, the founder of phrenology, was the first to use measurements of the head in order to predict personality traits. Gall, who called his theory craniology, divided the brain into 27 different areas, each with a specific function, such as affection, pride, religious feeling, poetical skill, and the tendency to murder. In the first half of the 19th century, phrenology's heyday, the theory was used to predict children's future and to screen job applicants, much as personality tests are sometimes used today.
In order to conduct a phrenological analysis, the scientist would measure the patient's head with a caliper, then feel the surface for raised and depressed areas. Raised areas were thought to indicate that the part of the brain located beneath that spot was well developed, while depressed areas indicated the opposite. Unfortunately, phrenology was sometimes used to promote racism, notably by the Nazis.
While phrenology has been superseded by neuroscience, psychology, and other modern scientific work on the brain, it is still well known and often referenced in popular culture, usually in a joking context. Many people are familiar with the look of a phrenology chart, which typically shows a head in profile with outlined areas denoting certain aspects of personality. The commonly heard phrase, "You ought to get your head examined," is actually a reference to phrenology, not to psychology, as is often assumed.